The most common and most likely answer to this crossword clue is the 5 letter word KERNE
K
E
R
N
E
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The straight part
We know that "Old soldier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old soldier = kerne.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does KERNE mean?
KERNE noun- Any part of a letter which extends into the space used by another letter.
- A light-armed foot soldier of the ancient militia of Ireland and Scotland; in archaic contexts often used as a term of contempt.
- A boor; a low person.
- An idler; a vagabond.